pre_commit hook overly expensive - should lazily compute parameters

[Robert Collins]

the Branch.pre_commit hook calculates a delta and revision tree object
that may not be needed or used, when a hook is present. This is partly
due to the signature - a Result (confusing name which this is before the
operation) - perhaps a PreCommitEvent? object should be passed into the
hook calls, and it can generate content on demand and cache it after
generating. This will make the debate about how shell hooks should fit
in simpler too.

 affects bzr
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